The terms “machine tool” and “screw machine” are used to describe two different items of machinery. These two types are similar in that they are used to effect a machining operation on a workpiece. They differ in that, in the machine tool, the tool is rotated relative to a stationary workpiece; whereas, in a screw machine, the workpiece is held in a rotatable spindle.
Some automatic screw machines have a plurality of workpiece-holding spindles journalled at circularly-spaced locations on a head, which is itself, in turn, rotatably mounted on a stationary body. In some of these machines, five such spindles are journalled on the head. Thus, the head may be rotatably indexed relative to the body between any of five separate index positions to position a rotating workpiece held in the spindle relative to one or more tools at that particular index station. However, it is necessary to provide a means for lubricating the head-mounted spindles.
U.S. Pat. No. 5,205,376 discloses a lubrication system for an automatic screw machine having a multi-spindle head rotatably mounted on a body. As shown in the '376 patent, annular grooves (indicated at 30, 32 in the '376 patent) extend inwardly from bearing surfaces (indicated at 26, 28, respectively) provided on the head. Passageways are provided through the body and communicate with the annular chamber formed by these head grooves and the facing body bearing surface. A plurality of passageways extend radially into the head from various locations along the groove, and communicate with the holes provided in the head in which the spindles are journalled. Thus, the lubricant flow path is from the external source through the body passageway and into the annular chamber surrounding the head, and from there via radial passageways in the head to the various spindle openings therein.
While the '376 patent does disclose an operable and functional lubrication system, it also has its problems. For example, by machining the annular lubrication groove into the head from a bearing surface, the remaining area of available bearing surface is reduced. In a region that is already prone to wear, the effective area between the head and body bearing surfaces maybe reduced by as much as about 25% because of the provision of this lubrication groove.
Second, the annular groove provides a long path for potential lubricant leakage. In some devices (e.g., the Davenport® Model B), the head bearing surface has a diameter of about 8.75 inches. Hence, the circumference (i.e., C=πd) is about 27.5 inches. Since the groove is positively pressurized relative to the atmosphere, and since there are potential leakage paths on both sides of the groove, the total length of the potential leakage path is about 55.0 inches. Thus, the '376 patent has a large path of potential lubricant leakage.
Third, the heaviest stock removal normally occurs at the two lowermost spindle positions (i.e., those proximate the 7:00 o'clock and 5:00 o'clock positions). In these two positions, the lubricant flow path is down and then up in the '376 patent. This can contribute to a “dry spindle” condition.
Finally, since the head rotates from one index position to another in a fraction of a second, it is believed to be unnecessary to lubricate the spindles continuously at all angular positions of the head relative to the body. Rather, it is thought to be sufficient to lubricate the spindles only when the head is at any of the various index positions. The oil film established at these various positions is virtually unaffected by a fraction-of-a-second travel time from one angular index position to the next. By contrast, in the '376 patent, all five spindles are lubricated simultaneously and continuously at all angular positions of the head relative to the body.
Additional details of prior art lubrication systems may be shown and described in U.S. Pats. Nos. 1,125,031, 2,245,105, 2,379,944, 2,625,448, 2,728,414, 3,351,392, 4,068,740, 4,344,507 and 4,905,794, and in Canadian Pat. No. 473,189.
Accordingly, it would be generally desirable to provide an improved lubrication system for a multi-spindle screw machine that overcomes these problems in the prior art.